The presently disclosed embodiments relate generally to a flexible electrophotographic imaging member having an anticurl back coating. The anticurl back coating of the flexible electrophotographic imaging member of the present disclosure not only provides wear/scratch resistance, it also gives the resulting imaging member flatness to meet the functional requirement of electrophotographic imaging apparatuses. While the present anticurl back coating (ACBC) can be used in all conventional electrophotographic imaging member designs, particular attention is focused on its application in a flexible multi-layered electrophotographic imaging member comprising a plasticized imaging layer.
In conventional prior art electrophotographic flexible imaging members, there may be included a photoconductive layer including a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes an imaging member having at least two electrically operative layers. One layer comprises a photoconductive layer or charge generating layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the charge genearting layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a charge generating layer.
In the case where the charge generating layer is sandwiched between the outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
Typical negatively charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer. The charge transport layer is usually the last layer, or the outermost layer, to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered imaging member material is obtained after finishing solution application of the charge transport layer coating and through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support. Since the charge transport layer in a typical imaging member has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature. Since the typical flexible electrophotographic imaging member, if unrestrained, exhibits undesirable upward imaging member curling, an anticurl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is necessary to provide the appropriate imaging member belt with desirable flatness.
Flexible electrophotographic imaging members having these electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
However, when a negatively charged imaging member (e.g., in belt configuration) is in dynamic cyclic motion under a normal machine operation condition in the field, the anticurl back coating of conventional imaging members (as the outermost exposed backing layer) is subject to high surface contact friction when it slides and flexes over the machine subsystems of the belt support module, such as rollers, stationary belt guiding components, and backer bars. The mechanical/frictional sliding interactions of ACBC against the belt support module components have been found to create numbers of issues; such as: (1) exacerbate ACBC wear/abrasion, causing loss of anti-curling control capability and resulting in imaging member belt curling-up problem because the thinning of the ACBC reduces its curl control effectiveness to result in premature curling up of the imaging member that impacts normal imaging belt machine functioning condition, such as non-uniform charging for proper latent image formation; (2) create debris/dirt of ACBC wear-off that scatters and deposits on critical machine components such as lenses; (3) wear/abrasion/scratch damage in the ACBC does also produce unbalanced forces between the charge transport layer and the ACBC to cause micro belt ripples formation during electrophotographic imaging processe; (4) cause the development of tribo-electrical charge built-up in the ACBC that increases belt drive torque and, in some instances, it has been found to result in belt stalling; (5) in other cases, the tribo-electrical charge build up can be so high as to cause sparking; and lastly (6) under extensively cycled condition in precision electrostatographic imaging machines, an audible squeaky sound generation due to high contact friction interaction between the ACBC and the backer bars has also been a problem. Therefore, pre-mature ACBC failure shortens the imaging member belt functional life and requires frequent costly belt replacement in the field. Moreover, inclusion of an ACBC to provide flatness also incurs additional material and labor cost.
To overcome the abovementioned shortcomings association with the conventional ACBC in the flexible imaging member belt, research activities devoted to ACBC elimination have been pursued and ACBC-free flexible imaging members have been designed. To achieve the purpose of ACBC elimination, these imaging members are re-designed so that they contain a plasticized charge transport layer (CTL) which minimizes the CTL/substrate dimensional contraction mismatch for effecting internal tension stress/strain build-up reduction in the CTL. Even though the ACBC-free imaging members provide valid curl reduction, they do not render the desirable member flatness and still exhibit about 16 inch to about 25 inch diameter of curl-up curvature. As used herein, the measurement of curvature is determined by the following: a 2 inch×10 inch sample was cut from an ACBC-free imaging member and left unrestrained and free standing on a table. The extent of sample upward curling was then measured and recorded as its diameter of curl-up curvature.
While the fabricated ACBC-free flexible imaging members having a plasticized CTL produce good photo-electrical functioning stability results, quality copy prints, and curl suppression, they are unable to provide the resulting imaging members with complete flat configuration to meet the high volume machines imaging member belt flatness requirement. Moreover, the unprotected bottom side of the substrate of these imaging members is highly susceptible to the development of pre-mature onset of wear/scratch failure against the machine belt module support rollers and backer bars sliding mechanical friction action under a normal dynamic belt cycling machine operation condition. This causes generation of large amount of debris and/or dust particles inside the machine cavity to adversely impede proper imaging member belt functional operation.
Thus, there exists a need to provide a flexible electrophotographic imaging member with an ACBC re-formulation that improves physical/mechanical function and does not suffer from the abovementioned issues while providing the imaging member flatness to meet machine functioning requirement.